Since rP23514 the game has the possibility to build structures automatically.
It was discussed via IRC 0ad-dev(11/Nov/21) to add this feature to the fields.
06:34 < wowgetoffmang> Freagarach: Stan Langbart : I think this feature could go into the base game for Farm Fields.
06:34 < Freagarach> Sounds nice indeed. :)
06:35 < wowgetoffmang> They would "grow" on their own.